1. Field of the Invention
This invention relates to investigating earth formations traversed by a borehole. More particularly, the present invention is directed to an apparatus and a method for determining element concentration values and for further characterizing the attributes of the formations surrounding a borehole.
2. The Related Art
The capital cost of drilling and evaluating a deep well, for example an oil or natural gas well, is extremely high, and for this reason considerable expense is incurred during those time intervals when drilling or production steps must be interrupted to evaluate the formation. With known analysis techniques, the concentration of some elements might be derivable from logging of the formation, but the concentrations of other elements would require the taking of core samples for analysis.
The existence and quantity of an element in a formation can be determined, as described in U.S. Pat. No. 3,665,195, by irradiating the formation with neutrons and detecting the induced gamma-ray activity from the element of interest. After determining the thermal neutron capture cross section of the formation, the product of the gamma-ray emission and the thermal neutron capture cross section is obtained as a quantitative indication of the element's abundance in the formation.
Hereafter is given a simplified view of the thermal neutron capture process. A burst of neutrons is created and propagate into the formation. Some of the neutrons are absorbed, but the majority slow down until they reach thermal energies. At thermal energies, the neutrons diffuse until they are captured by one of the nuclei of the formation. For a particular neutron, its capture will depend on the number of nuclei it "sees", weighted by the microscopic capture cross section (probability) of each nucleus (the total in a homogeneous formation is proportional to the formation capture cross section SIGMA.sub.form). Thus, the greater the number of nuclei of a particular element, the greater the number of neutrons that will be captured by that element. In other words, the number of capture gamma rays produced is, for a particular element, proportional to the number of nuclei per volume unit multiplied by the element cross section. Once the neutrons are captured they will produce a spectrum of gamma rays specific for each element. Some of these gamma rays interact with the detector and deposit all their energy or a fraction thereof, and others are lost. Those that are detected by the detector are used for the spectral measurement. This detected spectrum is decomposed to obtain the fractional contributions or yields, Y.sub.i, of each element in the total spectrum. Relative values for two Y.sub.i will be proportional to the relative atomic abundances of the elements in the formation (with the ratio weighted by many nuclear parameters, gamma-ray multiplicity, gamma-ray transmission probabilities, neutron capture cross sections, etc.). Once good relative yield measurements are provided, it is only necessary to determine the proper absolute normalization to transform these relative measurements into elemental concentrations.
In the article "The Aluminum Activation Log" by H. D. Scott and M. P. Smith published in 1973, there is described a method for measuring the aluminum content of the formation, in order to estimate the formation shale fraction. A californium-252 source of neutrons is used in conjunction with the formation thermal neutron capture cross section to produce a continuous activation log of a borehole.
The Al activation process is briefly hereafter described. In the neutron activation process, an atomic nucleus absorbs a neutron, creating an unstable isotope which decays, after some delay, usually by beta decay, with emission of associated gamma rays of characteristic energies. In aluminum activation, the natural isotope .sup.27 Al absorbs thermal neutrons and produces the unstable isotope .sup.28 Al, which beta decays with a half-life of 2.24 minutes, emitting a 1779 keV gamma ray. This sequence is summarized below: ##STR1##
The rays connected with the activation of different elements are capable of being separated, provided they have widely separated half-life characteristics. There are instances, however, where the bombardment by neutrons of two materials produces two isotopes having substantially the same half-lives and, in fact, there are instances where exactly the same radioisotope is produced from two different materials. For example, both .sup.27 Al and .sup.28 Si react with neutrons and produce .sup.28 Al which has a half-life of about 2.24 minutes. It is apparent that the determination of the amount of silicon and aluminum in a formation by bombarding the formation with neutrons is thus rendered difficult because the gamma-ray activity resulting when the .sup.28 Al atoms revert to their stable condition could not be separated into the components that are respectively due to aluminum and silicon.
As an attempt to overcome this difficulty, it has been proposed, as set forth in U.S. Pat. No. 3,156,822, to detect the resulting gamma-ray activity at a plurality of levels in the bore longitudinally spaced from the level of bombardment by distances such that the gamma-ray activities involving the different materials may be distinguished.
However, the tool disclosed in the '822 patent did not prove to be fully satisfactorily and its length is a major drawback, due to the numerous longitudinally spaced detectors.
U.S. Pat. No. 4,464,569 discloses a method for determining basic formation component volume fractions, including a spectroscopic analysis of capture gamma-ray spectra obtained from a neutron spectroscopy logging tool. The relative sensitivities of the logging tool to the specific minerals or to the chemical elements in the formation are determined either from core analysis or from tests run in known formations. The spectroscopic elemental yields and the relative sensitivities are then used together to determine the volume fractions of the basic formation components such as limestone, sandstone, porosity, salinity, dolomite, anhydrite, etc.
Nevertheless, the method described in the '569 patent does not require, and the patent does not disclose, a straight forward way for determining elemental concentrations, especially through the use of commonly available logging tools or modifications thereof. This known method takes appropriate combinations of measured yields, normalizes core data or laboratory measurements to obtain calibrated relative sensitivities and makes use of the constraint that the sum of all volume fractions is unity. Values of the volume fractions can then be found by solving the appropriate set of equations for the formation component volume fractions.
U.S. Pat. No. 4,810,876 contemplates a logging apparatus and processing methods for determining elemental concentrations, in order to assess the mineralogy of a formation, based on an indirect approach that in part relies upon certain unique assumptions.
This known method implies a substantial amount of calculation and includes some limitation due to the assumptions necessary to its implementation.
The article entitled "Geochemical Logging with Spectrometry Tools" by R. Hertzog et al., presented at the 62nd Annual Technical Conference and Exhibition of the SPE, held in Dallas, Tex. on Sept. 27-30, 1987, paper SPE 16792, discloses a Geochemical Logging Tool, known as GLT (mark of Schlumberger Technology Corporation), designed to measure natural, activation, and neutron capture gamma rays. The GLT tool produces logs of the most abundant elements and direct measurements of Al concentrations are provided. The GLT tool comprises a tool string including successively from top to bottom: (i) a natural gamma-ray tool, known as the NGS (mark of Schlumberger Technology Corporation) and depicted in U.S. Pat. No. 3,976,878; (ii) a source of low energy neutrons, preferably californium-252; (iii) an activation tool, known as AACT tool, adapted for measuring the gamma rays resulting from the activation of aluminum atoms by the neutrons emitted by the californium source; and (iv) a gamma spectrometer tool, known as the GST (trademark of Schlumberger Technology Corp.) and being such as depicted in U.S. Pat. Nos. 4,317,993 or 4,327,290; the GST tool is designed to detect gamma rays resulting from the capture of neutrons emitted by another source, i.e. a high energy (14 Mev) neutron generator provided in the string. The whole GLT tool involves three separate modes of gamma-ray spectroscopy to make a comprehensive elemental analysis of the formation. The first measurement is performed by the NGS tool which passes by the formation before any neutron source can induce radioactivity in order to derive the concentrations of K, Th, and U in the formation. The second measurement is performed by the AACT tool; the AACT tool, the NGS tool above it, and the .sup.252 Cf neutron source between them, allow a measurement of activation gamma rays to be used to derive formation aluminum concentration. The third measurement is performed by the GST tool to derive a spectrum of capture gamma rays from a plurality of elements in the formation, such as Si, Ca, Fe, S, Ti, K, and Gd. The GST tool uses a high energy (14 Mev pulsed neutron generator to induce these capture reactions.
Although the above mentioned GLT tool provides significant advantages over earlier tools, it is desirable to provide still further improvements.
Due to the relatively large number of devices composing the GLT string, the GLT turns out to be critically long.
Moreover, the safety concern with respect to the use of nuclear sources in boreholes, such as the californium source hereabove mentioned, has indubitably increased over the years. Accordingly, the regulations have become more and more stringent. For example, the activity (measured in Curies) of the sources should not exceed a given value. It is, however, difficult to determine and find a nuclear source which complies with the regulations as well as the needs of the industry.
Furthermore, the count rates related to the aluminum activation are relatively low, since the californium source has a relatively low neutron output.
Finally, since two different sources, i.e. the radioactive californium source and the high energy neutron generator are respectively used for the capture yields measurements and the aluminum activation measurements, an environmental correction for aluminum is required. Such correction is needed for taking into account the porosity and the absorption properties of the formation and of the borehole.
According to the above, there is a need for a logging tool for measuring natural, activation and neutron capture gamma rays which do not show the drawbacks hereabove mentioned.